Relatively thin materials, or tapes, have numerous applications that require the processing of relatively large amounts of the tape. For convenience, such tapes are frequently stored on reels both before and after being processed. For example, in thermal transfer printing, thermal transfer ribbon and receiving paper are drawn from separate supply reels through a printing station, where a thermal print head transfers ink from the ribbon to the paper. The printed paper and used ribbon are then loaded onto separate take-up reels for future disposition. Film projection similarly involves the transfer of a film of connected transparencies from a film supply reel through an optical projection station and onto a film take-up reel. Likewise, the paper-processing industry has numerous applications in which relatively thin paper tapes of widely varying width are stored on reels.
Regardless of the particular material involved, or its width, the relatively thin nature of the tape presents certain problems regarding its storage on reels or spools. For example, the processing of tape removed from a spool, or to be stored on another spool, typically occurs at a controlled rate which may be constant or changing. In applications in which the tape is stored on a supply reel before processing, motion of the reel must be controlled to closely correspond to the advance of the tape through the processing station. If the reel is free to pivot about its axis in an uncontrolled manner, slack will be introduced into the tape whenever an abrupt stop at the processing station is experienced. Then, when tape advancement through the processing station resumes, the inertia of PG,3 the reel will, at least initially, produce a relatively large amount of tension in the tape.
Similar problems occur when the tape is wound upon a take-up reel after exiting the processing station. For example, if the reel's motion and tape advancement through the processing station are independently controlled, the same type of tension discontinuities discussed above may result. In addition, if the advancement of tape through the processing station is controlled by motion of the take-up reel, the position of the tape with respect to the processing station is difficult to control.
Another factor complicating the transfer of tape from a supply reel, through a processing station, and onto a take-up reel in a particular desired manner, involves the changing radius of tape stored on each reel as tape is advanced through the processing station. More particularly, tape cannot be uniformly advanced through a processing station by uniformly rotating the takeup reel. As will be appreciated, when the tape is initially loaded onto the takeup reel, the radius of the reel and tape wound thereon is only slightly greater than that of the reel alone and the reel must travel at a relatively large angular velocity to advance the tape through the processing station at some predetermined rate. As more tape is wound upon the reel, however, the radius of the combined tape and reel increases and a smaller angular velocity of the reel produces the same tape advance rate through the processing station. As will be appreciated, operation of the supply reel presents the converse situation and the angular velocity of the supply reel must continuously be decreased as tape is transferred to the take-up reel, if a particular tape advance rate through the processing station is to be maintained.
Variations in the amount of material stored upon each reel also influence the maintenance of a desired tension in the tape as it is processed. More particularly, when the angular velocities of the supply and take-up reels are not properly adjusted in response to the transfer of tape therebetween, the tension in the tape may undergo substantial fluctuations. For example, if the supply reel is rotated too slowly, or the take-up reel too quickly, tape tension will increase. On the other hand, the tension in the tape may drop abruptly if the supply reel is rotated too quickly or the take-up reel too slowly.
As will be appreciated, the foregoing problems are frequently exacerbated by the relatively thin nature of the tape involved. For example, the thermal transfer ribbon employed in thermal transfer printing is typically on the order of six microns thick. Unlike tapes of thicker, heavier material, such transfer ribbons provide little resistance to the motion of the components that control the advance of the ribbon along a feed path. For that reason, the thermal transfer ribbon is highly susceptible to wrinkling and breakage when the controlling components allow the tape tension to undergo even minor fluctuations. The tape is simply unable to absorb variations in the operation of different system components.
A number of approaches have been taken to control the advance of a reel-stored tape through a feed path. For example, arrangements including a brake and slip clutch are frequently employed when the tape to be processed is received from an undriven supply reel and fed to a driven take-up reel. In such arrangements, a brake is held in frictional contact with a portion of the tape wound upon the supply reel. The brake introduces a back-tension into the tape and prevents the supply reel from overtraveling when the advance of the tape through the processing station is halted. The slip clutch controls the manner in which the take-up reel is driven, causing the reel to slip with respect to the driving element when the tension in the tape exceeds some predetermined level. As will be appreciated, by itself, this relatively simple system is unresponsive to the changing volume of tape wound upon the reels.
Another approach to the control of tape advanced along a feed path is disclosed in U.S. Pat. No. 4,000,804. In that arrangement, the tape progresses from a driven supply reel onto an independently driven take-up reel. The angular velocity of the supply reel is maintained substantially constant while tape is transferred between the reels, as is the torque applied to the take-up reel. Thus, although the tape tension and linear velocity of the tape between the reels varies, some tension is always maintained in the tape. The reference additionally notes, without discussion, that the operation of the motors controlling the reels can be varied as a function of the volume of tape on the spools.
The reel control system disclosed in U.S. Pat. No. 4,000,804, however, has several disadvantages. As noted, by maintaining the angular velocity of the supply reel, as well as the torque applied to the take-up reel, substantially constant, wide variations in the velocity and tension of the tape between the reels is experienced. Even if motor operation were adjusted to correct for these variations, the disclosed arrangement would be relatively ineffective if the processing of the tape that occurs between the reels frequently interrupts the tape's advance. More particularly, feedback corresponding only to tape build-up on the reel does not allow such fluctuating operating characteristics to be compensated for.
U.S. Pat. No. 4,025,830 discloses a web drive system for reversibly transferring a web between two reels. Web tension is maintained in one of two ways. In a first direction of web advance, tension is maintained by operating the reel upon which web material is being deposited at a greater speed than the other reel. When the direction of tape advance is reversed, however, web tension is maintained by applying greater torque to the reel upon which web material is being deposited, stalling the motor that drives the other reel. As will be appreciated, this arrangement also has the disadvantage of failing to compensate for the changing volume of tape wound on the reels.
U.S. Pat. No. 4,294,552 discloses a bidirectional ribbon drive control for transferring ribbon between a pair of reels. Ribbon tension is maintained by applying a drag torque to the reel that is currently supplying ribbon. This drag torque is overcome by a greater torque applied to the take-up reel. While the arrangement thus maintains some tension in the ribbon at all times, it does not compensate for changes in the volume of tape located on the reels.
Several systems have also been developed that clearly use feedback related to the changing volume of tape on a reel that occurs as the tape is processed. For example, U.S. Pat. No. 4,499,476 adds feedback to the brake-and-clutch control system described above. More particularly, electromechanical devices monitor the radius of the tape build-up on each reel and provide feedback signals to the electromagnetic brake and clutch. As a result, the braking force applied to the supply reel by the brake is directly proportional to the volume of tape stored on the reel. Similarly, the electromagnetic clutch is controlled to produce an approximately constant tension in the tape independent of the tape volume on the reel. As will be appreciated, however, this approach does not compensate for variations in tape tension that may result as the tape is started and stopped at a processing station.
Another arrangement employing feedback to control the advance of tape between a supply reel and take-up reel is disclosed in U.S. Pat. No. 4,479,081. In accordance with that arrangement, feedback relating to the angular velocity of the supply reel is used to adjust the angular velocity of the take-up reel to produce relatively uniform linear movement of the tape between the two reels.
While this arrangement does use feedback to control tape advance as a function of some varying system conditions, it does not monitor the tape's advance at a point between the reels. Thus, if nonuniform patterns of tape advance were created at a processing station located between the reels, the system would be unable to correct the resultant tension variations produced in the tape. In light of these considerations, it would be desirable to produce a system able to control the transfer of tape between a single reel and a processing station or between two reels interposed by a processing station, in a manner responsive both to changing tape volume on the reels and variations in the operation of the processing station without mechanical measuring devices such as rheostats.